Be in no doubt. If David Cameron becomes prime minister many will look back at the vote in Crewe and Nantwich as the moment they first believed it was possible.
Not only is this the first Tory by-election gain from Labour in 30 years, it is on a swing that matches those secured by Ted Heath and Margaret Thatcher before they reached No 10.
It comes less than a year after what was billed as Labour's moment of "renewal" when Gordon Brown succeeded Tony Blair. It comes just weeks after the prime minister responded to crushing defeats in the local elections by telling voters he felt their hurt before unveiling an unexpected tax cut and his legislative programme for the year ahead.
So, what is left to do now? A reshuffle? It would, most likely, be dismissed as moving the deckchairs on the Titanic. Ditching the captain then? Certainly that is being discussed by many Labour MPs - even some in the cabinet - but it stll looks unlikely - for now. A change of policy? That is where the debate will now focus with one senior minister telling me that it's time tear up what they referred to as "Gordon's five-year tractor plans" and take action now to ease the financial pain many are feeling - if necessary by postponing investment in hospitals and schools to pay for it.
Some in Labour will comfort themselves with the memory that Neil Kinnock was pretty good at winning by-elections but still couldn't win a general election. It is not likely to comfort very many.